The Great and Terrible implementation of MPI-2

function index


Frees a communication request object
int MPI_Request_free(
  MPI_Request *request


[in] communication request (handle)


Mark the request object for deallocation and set request to MPI_REQUEST_NULL. An ongoing communication that is associated with the request will be allowed to complete. The request will be deallocated only after its completion.

This routine is normally used to free inactive persistent requests created with either MPI_Recv_init or MPI_Send_init and friends. It is also permissible to free an active request. However, once freed, the request can no longer be used in a wait or test routine (e.g., MPI_Wait) to determine completion.

This routine may also be used to free a non-persistent requests such as those created with MPI_Irecv or MPI_Isend and friends. Like active persistent requests, once freed, the request can no longer be used with test/wait routines to determine completion.

Advice to users.

Once a request is freed by a call to MPI_REQUEST_FREE, it is not possible to check for the successful completion of the associated communication with calls to MPI_WAIT or MPI_TEST. Also, if an error occurs subsequently during the communication, an error code cannot be returned to the user --- such an error must be treated as fatal. Questions arise as to how one knows when the operations have completed when using MPI_REQUEST_FREE. Depending on the program logic, there may be other ways in which the program knows that certain operations have completed and this makes usage of MPI_REQUEST_FREE practical. For example, an active send request could be freed when the logic of the program is such that the receiver sends a reply to the message sent --- the arrival of the reply informs the sender that the send has completed and the send buffer can be reused. An active receive request should never be freed as the receiver will have no way to verify that the receive has completed and the receive buffer can be reused.

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.


All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

No error; MPI routine completed successfully.
Invalid MPI_Request. Either null or, in the case of a MPI_Start or MPI_Startall, not a persistent request.
Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).

See Also

also: MPI_Isend, MPI_Irecv, MPI_Issend, MPI_Ibsend, MPI_Irsend,
MPI_Recv_init, MPI_Send_init, MPI_Ssend_init, MPI_Rsend_init, MPI_Wait, MPI_Test, MPI_Waitall, MPI_Waitany, MPI_Waitsome, MPI_Testall, MPI_Testany, MPI_Testsome

Example Code

The following sample code illustrates MPI_Request_free.

#include "mpi.h"
#include <stdlib.h>
int main(int argc, char *argv[])
    MPI_Request r;
    MPI_Status s;
    int flag;
int buf[10];
    int rbuf[10];
    int tag = 27;
    int dest = 0;
    int rank, size, i;
    MPI_Init( &argc, &argv );
    MPI_Comm_size( MPI_COMM_WORLD, &size );
    MPI_Comm_rank( MPI_COMM_WORLD, &rank );
    /* Create a persistent send request */
    MPI_Send_init( buf, 10, MPI_INT, dest, tag, MPI_COMM_WORLD, &r );
    /* Use that request */
if (rank == 0) {
int i;
        MPI_Request *rr = (MPI_Request *)malloc(size *
for (i=0; i<size; i++) {
            MPI_Irecv( rbuf, 10, MPI_INT, i, tag, MPI_COMM_WORLD, &rr[i] );
        MPI_Start( &r );
        MPI_Wait( &r, &s );
        MPI_Waitall( size, rr, MPI_STATUSES_IGNORE );
    else {
        MPI_Start( &r );
        MPI_Wait( &r, &s );
    MPI_Request_free( &r );
    if (rank == 0)
        MPI_Request sr;
        /* Create a persistent receive request */
        MPI_Recv_init( rbuf, 10, MPI_INT, MPI_ANY_SOURCE, tag, MPI_COMM_WORLD, &r );
        MPI_Isend( buf, 10, MPI_INT, 0, tag, MPI_COMM_WORLD, &sr );
        for (i=0; i<size; i++) {
            MPI_Start( &r );
            MPI_Wait( &r, &s );
        MPI_Wait( &sr, &s );
        MPI_Request_free( &r );
else {
        MPI_Send( buf, 10, MPI_INT, 0, tag, MPI_COMM_WORLD );
return 0;